Fast Facts

The first Thescelosaurus specimens, collected in the late 19th century, sat in a crate for decades before they were rediscovered.

Size relative to a 6-ft (2-m) man:

This "beautiful but neglected lizard" was not recognized as unique until decades after the first fossils were collected.

Thescelosaurus neglectus was an upright runner that moved on two legs and used its long tail for balance. It remained relatively close to the ground and browsed low-lying vegetation.

Thescelosaurus has been at the heart of a prehistoric controversy because some have suggested the animal represents an evolutionary link between crocodilians and birds. A specimen nicknamed "Willo" may be the first dinosaur ever found with a fossilized heart. Some scientists further suggest that the organ is a four-chambered heart more like those of mammals or birds than those of reptiles.

Such a heart would help support the hypothesis that dinosaurs were warm-blooded, active animals more like today's birds or mammals than reptiles. However, whether the fossil even has a heart at all has been the subject of much debate. A number of scientists dispute the findings.

Thescelosaurus roamed western North America late in the reign of the dinosaurs, from perhaps as early as about 75 million years ago until the catastrophic Cretaceous-Tertiary extinction 65 million years ago.